Apple Senior Vice President of Worldwide Marketing Phil Schiller speaks about security features of the new iPhone 5S.

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Unlocking your iPhone with a fingerprint is convenient. But it could backfire if you end up in trouble with the law, warn some privacy experts.

The privacy quandaries stem as much from Apple’s technology as from unsettled U.S. case law when applied to the digital age.

Courts have given mixed messages about whether Americans are protected from being forced to divulge passwords or decrypt information for law enforcement officials. Civil liberties advocates argue defendants shouldn’t have to unlock their own computers for the cops. The logic: Under the Fifth Amendment, Police can’t force you to self-incriminate by testifying, or divulging something in your mind.

It’s unclear if that same protection applies if the password is your fingerprint.

“A fingerprint is entitled to less constitutional protection than a password known in your mind,” said Hanni Fakhoury, a staff attorney at the Electronic Frontier Foundation in San Francisco. “If police arrest you and ask you for a password, you could refuse and they’d be hard pressed to force you to divulge the password.”

Of course, police already collect fingerprints after booking a suspect. And the Supreme Court has also held that police don’t need a search warrant to collect fingerprints.

It isn’t hard to imagine police also forcing a suspect to put his thumb on his iPhone to take a look inside, said Brian Hayden Pascal, a research fellow at the University of California Hastings Law School’s Institute for Innovation Law.

“This puts a new kind of stress on the moment of arrest,” Mr. Pascal said. “Suddenly what looks like it would just be a boring part of an arrest now suddenly has all this amazing potential.”

An Apple spokesman declined to comment on the Fifth Amendment implications of Touch ID.

An iPhone fingerprint scan could theoretically create a new piece of police evidence, too. It’s possible the system could be used by police to better establish who was using a smartphone at a certain time. Federal investigators regularly use data about which phone number called another and when.

“The fact someone unlocked the phone could be used as circumstantial evidence to show dominion and control over the phone, much the same way fingerprints are used to prove someone had touched a gun or were at the scene of the crime,” Fakhoury said.